Ink jet printing systems generally are of two types: continuous stream or drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream through an orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a defined distance from the nozzle. At the break up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a specific location on a recording medium or to a waste ink collecting recipient (e.g. for recirculation). In drop-on-demand systems, a droplet is expelled from a nozzle to a position on a recording medium in accordance with digital data. A droplet is not formed or expelled unless it is to be placed on the recording medium.
The most important part of an ink used in ink jet printing are the dyes. Although a number of dyes have been proposed none has been produced which meets all the requirements of a modern printing process.
C.I. FoodBlack 2 (formula I) and dyes having similar structures have been mentioned as black dyes for ink jet inks in JP patent application Ser. No. 59-093,766. These dyes, however have a somewhat bluish hue and give images with poor light fastness. ##STR3##
Black dyes of formula (II) or mixtures thereof have been described for example in U.S. Pat. No. 4,626,284 ##STR4## wherein Q represents a phenylene or naphthalene group substituted by a group shown by SO.sub.3 M.sub.1 ; R represents a hydrogen atom, a lower alkyl group, a lower acylamino group, a nitro group or a halogen atom; R.sub.1 represents a phenyl group substituted by a group shown by SO.sub.3 M.sub.1 or a group shown by COOM.sub.1, a lower alkyl group substituted by COOM.sub.1, or a lower alkylaminocarbonyl lower alkyl group; R.sub.2 represents a hydrogen atom, a lower alkyl group substituted by a group shown by COOM.sub.1, or a lower alkylaminocarbonyl lower alkyl group; M.sub.1 represents an alkali metal atom or an ammonium cation; and n represents 0 or 1.
Black dyes of formula (III) or mixtures thereof have been described for example in U.S. Pat. No. 4,724,001 ##STR5## wherein Q represents a phenylene group substituted by a methyl group, a methoxy group or an ethoxy group or a naphthalene group substituted by SO.sub.3 M; X represents a hydrogen atom or a phenyl group substituted by SO.sub.3 M; m represents 1 or 2; n represents 1 when X represents a hydrogen atom and n represents 0 when X represents a phenyl group substituted by SO.sub.3 M, wherein X represents a hydrogen atom and m represents 1 when Q represents a naphthalene group substituted by SO.sub.3 M; and M represents an alkali metal atom or an ammonium cation.
Anionic black dyes of general formula (IV) are described in U.S. Pat. No. 5,053,495 ##STR6## where W is COOH; X is selected from the group consisting of hydrogen, COOH, SO.sub.3 H, halo, hydroxy, nitro, cyano, alkyl with 1 to 6 C atoms, alkoxy with 1 to 6 C atoms and acylamino with 1 to 6 C atoms; Y is hydrogen, COOH or SO.sub.3 H; Z is hydrogen, COOH or SO.sub.3 H, R and R.sup.1 are independently hydrogen or alkyl from 1 to 6 C atoms substituted by 0, 1 or 2 COOR.sup.2 groups; and R.sup.2 is hydrogen or alkyl from 1 to 6 C atoms provided that there are at least two COOH groups and that the number of COOH groups is equal to or greater than the number of SO.sub.3 H groups.
These dyes are claimed to give differential water solubility depending upon pH. On a plain paper substrate with a pH from 5 to 7 the water solubility of these dyes is much lower than at higher pH and therefore the dyes will be more resistant to removal from the substrate by washing.
Such dyes, however, give rise to a poor smudge behaviour due to bronzing (association of the dye on the surface of the recording material). Although they can provide pure blacks on plain paper or coated paper, these dyes may provide a somewhat bluish hue on other recording media.
Black dyes with improved water fastness of general formula (V) have been described in EP Patent Application 0,612,820 ##STR7## where R is NH.sub.4 and Y is hydrogen or SO.sub.3 R. These dyes, however, do not show a neutral hue and a yellow dye needs to be included in the black ink.
In EP Patent Application 0,597,672 black dyes of structure (VI) are described ##STR8## where R.sup.1 represents a phenyl or naphthyl group which may be substituted by a hydroxyl group, NH.sub.2, SO.sub.3 M or COOM wherein M represents a hydrogen atom, or a cation species derived from an alkali metal, ammonia or an amine; R.sup.2 represents a carboxyalkyl group wherein the carboxy group may form a salt with an alkali metal or ammonia, a substituted or unsubstituted alkoxyalkyl group or a substituted or unsubstituted phenyl or alkanoyl group, and R.sup.3, R.sup.4 and R.sup.5, which may be the same or different, each independently represents a hydrogen atom or SO.sub.3 M wherein M is defined as above, provided that R.sup.1 does not represent a phenyl group substituted with SO.sub.3 M when R.sup.3, R.sup.4 and R.sup.5 all represent SO.sub.3 M.
These dyes, however, do not show a neutral black hue and a yellow dye needs to be included in the black ink.
Of the compounds represented by the general formula (VI) wherein R.sup.1 represents a phenyl group substituted by SO.sub.3 H and where R.sup.3, R.sup.4 and R.sup.5 all represent SO.sub.3 H only a few dyes having particular structures can be combined with other dyes, because other combinations deteriorate the storage stability of the inks or the light fastness of the prints.
Black disazo dyes of formula (VII) are described in GB Patent Application 2,289,473 and WO Patent Application 95/31505 ##STR9## wherein R.sub.1 is optionally substituted phenyl or optionally substituted naphthyl; R.sub.2 is hydrogen, halo, COOH, SO.sub.3 H, optionally substituted alkyl, optionally substituted alkoxy or optionally substituted alkylthio; R.sub.3 is NR.sub.7 R.sub.8 ; R.sub.4 is hydrogen or optionally substituted alkyl; R.sub.5 is hydrogen, optionally substituted alkyl or phenyl; R6 is hydrogen or SO.sub.3 H; R.sub.7 is hydrogen or optionally substituted alkyl, R.sub.8 is optionally substituted acyl.
Black disazo dyes of formula (VII) are described in JP Patent Application 07-268,256 where R.sub.1 is optionally substituted phenyl, R.sub.2 is branched alkyl or branched alkoxy with 1 to 8 C atoms; R.sub.3 is alkoxy with 1 to 9 C atoms; R.sub.4 is hydrogen, optionally substituted phenyl, CH.sub.2 COOH or C.sub.2 H.sub.4 COOH; R.sub.5 is hydrogen and R.sub.6 is hydrogen or SO.sub.3 H.
Black disazo dyes of formula (VII) where R.sub.1 is carboxyphenyl; R.sub.2 and R.sub.3 are hydrogen, methyl or methoxy; R.sub.4 is hydrogen; R.sub.5 is phenyl or carboxyphenyl and R.sub.6 is hydrogen are described in JP Patent Application 04-304,274.
Black disazo dyes of formula (VII), where R.sub.1 is optionally substituted carboxyphenyl; R.sub.2 is alkyl or alkoxy with 1 to 4 C atoms or CH.sub.2 COOH; R.sub.3 is alkoxy with 1 to 4 C atoms; R.sub.4 is hydrogen; R.sub.5 is hydrogen, alkyl with 1 to 4 C atoms or CH.sub.2 COOH and R.sub.6 is hydrogen are described in U.S. Pat. No. 5,198,022. All these dyes are to be substituted by one or more carboxy or carboxy alkyl groups, preferably they contain at least as many COOH groups as SO.sub.3 H groups.
Dyes containing COOH groups however tend to cause "crusting" resulting in a "poor first ejection" according to EP Patent Application 0,612,820.
Dyes of the above mentioned structures considered to be state of the art do not satisfy all the required demands when used in aqueous inks for ink jet printing.
Dyes used for such recording liquids have to be sufficiently soluble, do not show bronzing on recording materials (association of the dye on the surface of the recording material), they have to be stable even when the recording liquid is stored for a long period of time, they need to provide printed images having high optical density, excellent water fastness and excellent light fastness.
Various types of compositions have been proposed as inks for use in ink jet printing. Typical inks are composed of various types of dyes, water, organic cosolvents and other additives.
In particular the inks have to meet the following criteria:
(1) The ink gives high quality images on any type of recording medium. PA1 (2) The ink gives printed images exhibiting excellent water fastness. PA1 (3) The ink gives printed images exhibiting excellent light fastness. PA1 (4) The ink does not clog jetting nozzles even when these are kept uncapped while recording is suspended. PA1 (5) The ink is quickly fixed onto recording media without bronzing and can not therefore be scrubbed off. PA1 (6) The ink can be stored for a long period of time without deterioration in its performance. PA1 (7) Physical properties of the inks, such as viscosity, conductivity and surface tension, are each within a defined range. PA1 (8) The recording liquid has to be safe, non toxic and not flammable. PA1 R.sub.1 is hydrogen, alkyl, optionally substituted alkyl, alkoxy, optionally substituted alkoxy, thioalkoxy, optionally substituted thioalkoxy, halo, COOM or SO.sub.3 M; PA1 R.sub.2 is alkyl, optionally substituted alkyl, alkoxy or optionally substituted alkoxy; PA1 n is 2 to 4; and PA1 M is hydrogen, a metal atom, preferentially an alkali metal atom, ammonium or ammonium substituted with alkyl, alkoxyalkyl or hydroxyalkyl each having 1 to 12 C atoms.
Reactive black dyes have also been proposed for ink jet printing for example in EP Patent Application 0,693,537. However, inks containing dyes with incorporated reactive groups deteriorate the storage stability of the inks and due to their potential health risk they are less suitable for handling in a non chemical environment.